Aquifer recharge from the 1969 and 1978 floods in the Mojave River basin, California
The Mojave River basin, a high desert area in southwestern San Bernardino County, Calif., received 2.3 times the normal annual precipitation during the 1969 and 1978 water years. Precipitation in the mountainous upper part of the watershed is the primary source of flow in the Mojave River. Total precipitation at Lake Arrowhead, representative of the mountainous area, was 98 inches in the 1969 water year and 93 inches in the 1978 water year. Of these totals, 94 inches in 1969 and 88 inches in 1978 fell during the rainy season, December through April. The resulting flood period in 1969 produced an instantaneous peak discharge of 18,000 cubic feet per second at Afton, about 100 miles downstream from Lake Arrowhead. This discharge had an approximate flood-recurrence interval of 30 years. An instantaneous peak of 24,800 cubic feet per second was measured during the 1978 floods at Deep Creek. This discharge had an approximate flood-recurrence interval of 20 years. A comparison of the hydrologic data for the 1969 and 1978 flood periods indicates that although more precipitation occurred in 1969, more recharge occurred in 1978. The factors that caused the greater recharge were: (1) The more evenly distributed precipitation from December 1977 to April 1978, allowing for more uniform surface-water runoff in the Mojave River; (2) the dams constructed in the upper basin after 1969, which regulated floodflow peaks and allowed more water to stay in the basin; and (3) the lower water level in the aquifer in 1978, which made more space available to store the recharge water. Total recharge resulting from the floods is estimated to have been 245,000 acre-feet in the 1969 water year and 282,000 acre-feet in 1978. At present (1979) costs of water from Silverwood Lake, the 1969 recharge would be worth about $4.6 million and the 1978 recharge about $5.2 million. If the water were derived from the turnout of the California Aqueduct, the undelivered costs would be about $4.9 million for 1969 and $5.6 million for 1978.